1. Field of the Invention
This invention relates to a radiation image read-out method and apparatus. This invention particularly relates to a radiation image read-out method and apparatus, wherein two image signals are respectively detected from the front and back surfaces of a stimulable phosphor sheet, on which a radiation image has been stored, and the two image signals are then added to each other in a predetermined addition ratio, and wherein a change of the addition ratio with the passage of time is compensated for appropriately.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
For use in radiation image recording and reproducing systems, techniques for carrying out superposition processing on radiation images have heretofore been proposed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object be detected accurately in the radiation image. The extent to which such differences in the radiation energy absorption characteristics can be detected in a radiation image is referred to as the contrast detection performance or, simply, as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool, in particularly, in the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image be enhanced. Practically, the detection performance is adversely affected by various noises.
For example, in the aforesaid radiation image recording and reproducing systems using stimulable phosphor sheets, it has been found that the noises described below occur during the step of recording the radiation image on the stimulable phosphor sheet, and of reading out the radiation image therefrom.
Such noises include
(1) A quantum noise of radiation produced by a radiation source. PA1 (2) A noise due to nonuniformity in how a stimulable phosphor coated on the stimulable phosphor sheet is distributed, or how stimulable phosphor grains are distributed on the stimulable phosphor sheet. PA1 (3) A noise of stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. PA1 (4) A noise of light, which is emitted by the stimulable phosphor sheet, guided and detected. PA1 (5) An electric noise in the system for amplifying and processing an electric signal. PA1 i) exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which have been produced by a stimulating ray source and cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, PA1 ii) photoelectrically detecting the light, which is emitted from the front surface of the stimulable phosphor sheet, and the light, which is emitted from the back surface of the stimulable phosphor sheet, respectively with two photoelectric read-out means, two image signals being thereby obtained, and PA1 iii) adding the two image signals to each other in a predetermined addition ratio, an addition image signal, which represents the radiation image, being thereby obtained, PA1 wherein the improvement comprises the steps of: PA1 i) a means for exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which have been produced by a stimulating ray source and cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, PA1 ii) two photoelectric read-out means for respectively photoelectrically detecting the light, which is emitted from the front surface of the stimulable phosphor sheet, and the light, which is emitted from the back surface of the stimulable phosphor sheet, two image signals being thereby obtained, and PA1 iii) a means for adding the two image signals to each other in a predetermined addition ratio, an addition image signal, which represents the radiation image, being thereby obtained, PA1 wherein the improvement comprises the provision of: PA1 i) exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which have been produced by a stimulating ray source and cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, PA1 ii) photoelectrically detecting the light, which is emitted from the front surface of the stimulable phosphor sheet, and the light, which is emitted from the back surface of the stimulable phosphor sheet, respectively with two photoelectric read-out means, two image signals being thereby obtained, and PA1 iii) adding the two image signals to each other in a predetermined addition ratio, an addition image signal, which represents the radiation image, being thereby obtained, PA1 wherein the improvement comprises the steps of: PA1 i) a means for exposing a stimulable phosphor sheet, on which a radiation image has been stored, to stimulating rays, which have been produced by a stimulating ray source and cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation, PA1 ii) two photoelectric read-out means for respectively photoelectrically detecting the light, which is emitted from the front surface of the stimulable phosphor sheet, and the light, which is emitted from the back surface of the stimulable phosphor sheet, two image signals being thereby obtained, and PA1 iii) a means for adding the two image signals to each other in a predetermined addition ratio, an addition image signal, which represents the radiation image, being thereby obtained, PA1 wherein the improvement comprises the provision of:
Superposition processing is carried out in order to reduce the aforesaid noises markedly, so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally (i.e. the detection performance of the radiation image can be improved markedly improved). Ordinary techniques and effects of the superposition processing are as described below.
Specifically, radiation images are recorded on a plurality of recording media, which overlap one upon another. A plurality of image signals are detected from the plurality of the recording media and then superposed one upon another (i.e., added to one another). In this manner, various noises described above can be reduced. Specifically, an image signal having a high signal-to-noise ratio can be obtained.
Also, it is possible to employ a technique, wherein two image signals are detected from the opposite surfaces of the stimulable phosphor sheet, and the image signal components of the two image signals are then added together, which image signal components represent corresponding picture elements on the front and back surfaces of the stimulable phosphor sheet. Such a technique is proposed in, for example, U.S. Pat. No. 4,346,295. In such cases, the image information, which has been recorded with the radiation absorbed by the portion of the stimulable phosphor layer located on the radiation incidence side, is primarily obtained from the surface of the stimulable phosphor sheet, which surface was located on the radiation incidence side when the stimulable phosphor sheet was exposed to the radiation during the image recording operation. Also, the image information, which has been recorded with the radiation absorbed by the portion of the stimulable phosphor layer located on the side opposite to the radiation incidence side, is primarily obtained from the surface of the stimulable phosphor sheet, which surface was located on the side opposite to the radiation incidence side when the stimulable phosphor sheet was exposed to the radiation during the image recording operation.
When two image signals detected from the opposite surfaces of the stimulable phosphor sheet are superposed one upon the other, a superposition image signal can be obtained which reflects a larger amount of image information. Specifically, an image signal having a higher signal-to-noise ratio (S/N) can be obtained.
In cases where the operation for detecting two image signals from the opposite surfaces of a stimulable phosphor sheet is carried out, and a visible image is reproduced from an addition signal, which has been obtained by adding the two image signals to each other, noise reduces in the visible image as a whole. However, the sharpness of the visible image also becomes low. Therefore, for example, in cases where an image region of a high spatial frequency is to be used and therefore is required to have a good image quality, signal processing should preferably be carried out by using only the image signal, which is detected from the front surface side of the stimulable phosphor sheet, such that the image may have good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. In cases where an image region of a low spatial frequency is to be used and therefore is required to have a good image quality, it is not necessary for the region of a high spatial frequency to have high sharpness in the reproduced image, and noise in the image as a whole should be reduced such that the image may have good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in such cases, the superposition processing described above may be carried out.
As described above, the spatial frequency of interest in the image to be used varies for different kinds of images. Therefore, if the superposition processing is merely carried out, the problem will occur in that appropriate addition processing cannot be carried out. Also, noise components vary for different doses of radiation irradiated to the stimulable phosphor sheet and for different kinds of the stimulable phosphor sheets. Therefore, the addition ratio, in which the two image signals are added to each other and which yields the highest signal-to-noise ratio, varies for different doses of radiation and different kinds of the stimulable phosphor sheets.
Accordingly, the applicant proposed a novel superposition processing method for a radiation image in U.S. Pat. No. 5,477,059. With the proposed method, in cases where an operation for detecting two image signals from the opposite surfaces of a stimulable phosphor sheet is carried out, and the two image signals are then added to each other, an addition ratio, in which the two image signals are added to each other, is changed in accordance with the kind of the stimulable phosphor sheet, the spatial frequency of interest in the radiation image, the conditions under which the radiation was irradiated, and the like. In this manner, a superposition image signal can be obtained such that an image, in which the signal-to-noise ratio at the spatial frequency of interest is highest, may be reproduced regardless of the kind of the image to be used, the dose of radiation irradiated to the stimulable phosphor sheet, or the like.
However, even if the optimum addition ratio yielding a reproduced visible image, in which the signal-to-noise ratio is highest, is obtained, the optimum addition ratio will often change with the passage of time due to a change in the read-out gain of each photoelectric read-out means, the output power of the stimulating rays, or the like. If the addition ratio thus changes, the signal-to-noise ratio of the reproduced visible image will become low. Therefore, a reproduced visible image cannot be obtained which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.